1. Field of the Invention
The present invention relates to a process for preparing the controlled-release chitosan microcapsule, more specifically to a process for preparing the controlled-release chitosan microcapsule capable of releasing at an appropriate rate for a long time, which is characterized by:
(a) Biodegradable chitosan polymer is used for the capsule material; PA1 (b) The capsule is kept stable during the release time by the rapid and effective crosslinking reaction between the chitosan polymer and the double crosslinking agent on the surface of fine particles formed by a new emulsion interface reaction method using a double crosslinking agent of sulfuric acid and glutaraldehyde; and PA1 (c) The microcapsule which is smaller than several .mu.m s can be easily formed and the capsule is kept stable for a long release time because an insoluble polymer film whose source material is biodegradable is formed after the release. PA1 (a) A stage wherein emulsion 1 is prepared by inserting an aqueous polymer solution containing 0.3-10 wt % of chitosan and 0.1-2 wt % of acetic acid to an emulsion; PA1 (b) A stage wherein emulsion 2 is prepared by inserting an aqueous crosslinking agent solution containing 0.5-10 volume % of sulfuric acid and 1-50 volume % of aqueous 25% glutaraldehyde solution to an emulsion; and PA1 (c) A stage wherein the said emulsion 1 and emulsion 2 are mixed 1:1 under high-speed agitation, and a microcapsule is formed by the emulsion interfacial reaction between chitosan polymer and the crosslinking agent. PA1 (a) The contents and the chitosan polymer solution which is the capsule material enclosing the contents, are dispersed as fine particles in a nonsolvent medium under high-speed agitation, and stabilized with the aid of emulsifying agent; and PA1 (b) An insoluble polymer film is formed by the interfacial reaction with a crosslinking agent on the particle surface, and the contents are enclosed by this polymer film. PA1 (a) A process of preparing emulsion 1 containing chitosan polymer; PA1 (b) A process of preparing emulsion 2 containing a crosslinking agent; PA1 (c) A process of forming microcapsule by mixing the said emulsion 1 and emulsion 2; and PA1 (d) A process of washing and drying the prepared microcapsule. PA1 (a) The capsulation is rapid; PA1 (b) The process is pro-environmental because water is used as a solvent; PA1 (c) The capsule size and its distribution can be controlled by controlling the agitating speed, amount of emulsifying agent and the concentration of polymer in the aqueous solution; and PA1 (d) The release rate of the contents can be controlled by controlling the crosslinking of the polymer film with the amount of the emulsifying agent.
2. Detailed Description of the Related Arts
From the 1960s, researchers have been carried our for new types of controlled release systems maintaining effective agrochemical concentration for pest control by releasing agrochemicals at a pertinent rate for the best pharmacological effect with the least amount of agrochemicals. This researchers have been focused on solving the problems caused by the fact that the conventional agrochemical spraying does not provide the specificity of the solution-absorption and the transfer process, for example the danger of poisoning caused by the skin contact due to the direct spraying, the waste of agrochemicals by excessive spraying, environmental pollution and soil acidification, and so on.
Among these controlled release system, the microcapsule is the most effective system owing to its diverse applications. Especially in medicine, food and agrochemicals, the use of the microcapsule is increasing rapidly.
Microcapsule is a kind of small container which encloses the contents with film. Mainly, the film is made of synthetic of natural polymer, and controls the release of the contents. The releasing rate of the contents in a microcapsule is mainly determined by the chemical structure, thickness of the capsule film and the size of the microcapsule.
The process for preparing microcapsules is roughly divided into physical coating, phase separation and interfacial polymerization. In physical coating and phase separation, the contents are capsulated by using the conventional polymers. In interfacial polymerization, microcapsule is prepared by dispersing or emulsifying the contents in the immiscible medium and forming the film by the interfacial polymerization of the monomers on the dispersed phase surface. Capsule can also be prepared by the interface reaction [J. Appl. Polym. Sci., 59 (1996) 1795-1801] which is similar to the interface polymerization, wherein the polymer solution containing the contents is inserted into the medium containing a crosslinking agent reactive to the polymer, an insoluble film is formed by the fast crosslinking reaction on the particle surface between the polymer and the crosslinking agent, and a stable capsule is prepared.
Generally, as a hydrophilic polymer has loose structure in moist condition due to the swelling caused by water absorption, the release of the contents is performed easily if is used for the material of the capsule film. Also, as the release rate can be controlled by the crosslinking density of the capsule film, the hydrophilic polymer is suitable for the material of capsule for the agrochemical use. Polyvinyl alcohol [J. Appl. Polym. Sci., 60 (1996) 9-20], polyacrylic acid [J. Appl. Polym. Sci., 41 (1990) 2133-2145], sodium alginate [J. Appl. Polym. Sci., 43 (1991) 2123-2135] and chitosan [J. Appl. Polym. Sci., 51 (1994) 1427-1432] are known as the hydrophilic polymer suitable for the capsule material.
If the capsule material is not decomposed and remains after the contents are completely released, it can be another pollution source. Among the known soluble polymers mentioned above, chitosan is a cationic polymer with relatively good biodegradability. Recently, chitosan is known as improving the soil productivity and having a very positive effect on the life growth.
Preparation of microcapsule by using chitosan is a kind of an interface reaction. Rather than using chitosan only, aqueous solution of sodium alginate, containing target material which is anionic polymer, is dropped into the chitosan solution in the form of regular-size sphere. Then there occurs an ion-complex reaction between the anoinic and the cationic polymer on the interface, and an insoluble film forms on the surface, thereby the capsule is prepared. However, the size of the capsule prepared from this interfacial polymerization method is relatively large--from tens to hundreds of .mu.ms. The result is the increased capsule loss due to the decrease of the adhesion to the plant surface, when used for agrochemicals. In addition, although the chitosan has advantages for capsule material of agrochemicals as mentioned above, its utilization is limited because it is not easy to make microcapsules made of chitosan only. For this reason, it is necessary to make micro-sized chitosan capsules.